Hydraulic performance assessment of Denil fishway with modified bed slope and baffle spacing

· The hydraulic performance of the Denil Fishway is investigated through numerical CFD simulations. · Denil fishways with larger baffle spacing have delightful performance based on larger discharge capacity but unfavorable efficiency due to reduction of low-velocity and low-TKE zone. · With various baffle spacings, the Denil with S = 0.3 m are more optimized for passing different fish. · Denil fishways with increased baffle slope have better water permeability which is favorite; but in contrast, larger magnitude of the flow velocity within the fishway along with higher TKE values, makes it harder for fish to reach upstream. · The Denil fishway with a slope of 10 % was evaluated as the most optimal among the three proposed channel slopes. A Denil fishway is an eco-hydraulic structure designed to facilitate fish migration through river obstacles, enabling fish to ascend against the main current. This study evaluates the hydraulic performance of a conventional Denil fishway, with a focus on key parameters such as discharge rate, velocity distribution, and turbulence characteristics. The evaluation was conducted under varying baffle spacings ( S = 0.3, 0.4, and 0.5 m) and bed slopes (S₀ = 0.05, 0.1, and 0.15). Computational Fluid Dynamics (CFD) simulations were performed using laboratory data derived from a comprehensive literature review. The results reveal that increasing either the baffle spacing or the fishway slope leads to an increase in discharge capacity, which is beneficial. However, this also results in higher average velocities along the fishway, an increase in turbulent kinetic energy (TKE), and a reduction in low-velocity and low-TKE zones in fish resting areas, which negatively impacts the functionality of the fishway. Consequently, larger baffle spacings or steeper channel slopes make it more challenging for fish to swim or rest. Additionally, while the turbulence dissipation (TD) increases, it does not compensate for the rise in TKE. Although hydraulic data alone cannot provide a comprehensive evaluation of the efficiency of fishways without ecological data or field studies, this study highlights the significant influence of baffle spacing and bed slope on the hydrodynamic properties of Denil fishways. From a hydraulic perspective, the optimal design was identified with a baffle spacing of 0.3 m and a bed slope of 0.1.